In manufacturing a component to be incorporated into a precision equipment, such as semiconductor integrated circuit, the advanced technology such as photolithography or nanoimprint lithography is often employed. On use of such technology, it is important that substrates have as few defects as possible on their surface. For instance, if a photomask used in the photolithography as an original for exposure bears defects, there is a risk of forming a defective pattern because the defects are directly transferred to the pattern. To accommodate a need for ultra-miniaturization of patterns as found in the latest EUV lithography, the substrate serving as the original is also required to have a flat, smooth, substantially defect-free surface.
Synthetic quartz glass substrates for use as photomasks and liquid crystal filters must have high flatness, high smoothness, and low defectiveness. They are subjected to several resurfacing steps including lapping and polishing steps before they are ready for use in the subsequent process. The lapping step is to remove work strains introduced by slicing from an ingot. The polishing step is to mirror finish the substrate for modifying the flatness and shape of its surface. The final polishing step is to polish the substrate with colloidal silica abrasive having a small particle size, obtaining a substrate with a flat and smooth surface and devoid of microscopic defects.
For example, Patent Document 1 describes a method of acquiring a least defective substrate wherein the final polishing step includes a polishing step using a polishing pad having a nap layer made of ester resin and colloidal silica having a small particle size. Patent Document 2 discloses a method of lapping a large-size substrate using a lapping plate which is provided with grooves for allowing a flow rate of slurry. Further, Patent Document 3 describes a method of polishing a substrate so as to reduce defects wherein the final polishing step uses an expanded urethane suede polishing pad having a nap layer provided with grooves of predetermined depth.
The synthetic quartz glass substrates which can comply with not only the ArF excimer laser lithography, but also the EUV lithography are required to have a minimal number of defects on their surface. In the event of large-size substrates which are normally difficult to provide a slurry supply, it is required that the slurry be sufficiently distributed over the substrate so as to minimize the number of defects on substrate surface.
Although the method of Patent Document 1 is satisfactory as a general process when photomask substrates having a line width of down to 45 nm are manufactured, it is difficult to manufacture ultra-low defective substrates free of defects having a major diameter of the order of 40 nm. Even if such ultra-low defective substrates can be manufactured, the manufacture yield is very low. As long as the polishing pad and slurry are fresh, they may have full abilities to perform in the final polishing step. Once either one begins degradation, the balance of polishing conditions is quickly broken. Particularly in the event of large-size substrates, the slurry thickens and gels so that the slurry may not be distributed throughout the substrate, causing more defects to the substrate surface and negating the long-term service.
Although the method of Patent Document 2 is effective in distributing the slurry throughout the substrate, there is a concern that the flatness of lapped substrates be exacerbated by a certain factor like a phenomenon that the lapping plate will be deformed with the lapse of time, since the lapping plate itself is provided with grooves. It is expectable that substrates with a minimal number of defects can be manufactured by the method of Patent Document 3 as long as the lapping pad is fresh. For the reasons including wear of the nap layer, deformation of groove shape, and reduction of groove depth which will occur during successive operation of the polishing pad, it is deemed difficult to acquire least defective substrates in a consistent manner.